Measuring 125 to 170 centimetres (49 to 67 in) in length, this large swan is wholly white in plumage with an orange bill bordered with black. It is recognisable by its pronounced knob atop the bill.

When swimming, the mute swan holds its neck in a graceful "S" curve with its bill pointing downward. The whooper swans usually carry their bill level with their neck erect. The male frequently arches his secondary wing feathers over his back in an aggressive display posture.